Adipic acid (AA) is a raw material used in the production of many different materials including organic polymers (most notably Nylon-6,6), fibers, plasticizers, and food additives. The manufacture of these materials requires millions of tons of highly pure adipic acid each year.
Several methods of producing adipic acid are known. The currently predominant industrial process is performed in two separate steps. The first step is oxidizing cyclohexane to a mixture of cyclohexanone and cyclohexanol (KA mixture), and the second step is converting the KA mixture to adipic acid. The second step is typically performed using concentrated nitric acid (.about.55 wt % in the reaction) as an oxidant. Unfortunately, the nitric acid oxidation step results in the production of NO.sub.x (especially N.sub.2 O) byproducts that can pollute the atmosphere and are not readily recyclable.
One step processes for preparing adipic acid by air or peroxide oxidation of cyclohexane have been reported. See, e.g., U.S. Pat. Nos. 5,221,800; 5,929,277; and Catalysis Today, 9: 237, 1991. These processes are typically performed using a Co(III) catalyst at high oxygen pressure (e.g., 20-30 atm) or N-hydroxy-phthalimide/Co/Mn catalysts at low O.sub.2 pressure (Iwahama, T.; Syojyo, K.; Sakaguchi, S.; Ishii, Y. Organic Proc. Res. Devel. 1998, 2, 255-260). Despite the potential efficiencies and cost savings associated with such one step processes, the two step process continues to be preferred in the industry because conventional one step processes have not been optimized for large scale syntheses. For example, conventional one step processes using a Co-based catalyst require that a very high concentration (e.g., about 0.01 M) of the catalyst be included in the reaction mixture. As this catalyst is relatively expensive, cost considerations mandate that it be recycled using an extraction procedure prior to reuse in additional runs. Conventional one step processes also offer relatively low selectivity, and result in an adipic acid product of relatively low purity (e.g., less than about 70% pure). For these reasons, cobalt-catalyzed one step oxidation processes generally involve costly purification/recycling steps for purifying the adipic acid from by-products of the reaction, and for recycling the catalyst.